Den seismiske datainnsamlingen basert på bruk av noder på havbunnen var i sin tid den største i sitt slag i verden.
Nå melder TGS at de sammen med partnerne Aquila Holdings ASA og Viridien (tidl. CGG) har reprosessert dataene fra Utsira OBN Survey.
Reprosesseringsprosjektet har gitt betydelige forbedringer i oppløsning og frekvensinnhold for tolkning av forkastninger og arbeidsflyt for reservoarkarakterisering, skriver TGS.
Selskapet påpeker at prosjektet har fått betydelig industristøtte da etterspørselen etter høyoppløselig seismikk er stor i dette svært prospektive området.
Utsira OBN ble innsamlet i 2018 og 2019 av TGS og Axxis Geo Solutions og dekker om lag 2 000 km2 vest for Utsirahøyden i den sørlige Vikinggrabenen (se kartet under).
I 2023 ble Utsira OBN Survey nominert til prisen Exploration Innovation Prize 2023, som deles ut av GeoPublishing under NCS Exploration-konferansene i Oslo (vinneren ble Aker BP).
Begrunnelsen for Utsira OBN survey var som følgende:
Ocean bottom node (OBN) is the ultimate imaging tool to help operators overcome complex geological imaging challenges.
Exploration on the Norwegian Continental Shelf is in a mature phase, resulting in a growing need for higher-quality seismic data to locate the remaining, smaller hydrocarbon accumulations.
At the time of acquisition, the Utsira OBN (ocean bottom node) survey in the North Sea was the largest multi-client survey of its kind in the world.
Targeting approximately 2.000 km2 west of the Utsira High, the survey has delivered 3D seismic data in a prolific area not far from fields such as Johan Sverdrup, Edvard Grieg, Gudrun, and Gina Krog, along with several undeveloped discoveries and prospects in the Southern Viking Graben in the North Sea.
Utsira OBN was acquired by TGS and Axxis Geo Solutions (AGS) in 2018 and 2019. The survey realized dense sampling and high imaging quality using full azimuth and ultralong offsets. The offsets recorded in full azimuth were up to 43 km. The raw data weight was 1.5 petabytes, and the data gave a fold (a measurement of overlapping) of up to 1.300, whereas normal streamer 3D typically has 80.
According to TGS and AGS, the survey is of value not only for exploration but also for the delineation of discoveries, field development, and reservoir management.
Ocean bottom acquisition provides several benefits compared to conventional streamer acquisition.
The acquisition can either be done by ocean bottom cable, or, as in the case of Utsira, ocean bottom nodes.
Ocean bottom acquisition may be more expensive than conventional streamer acquisition but provides several benefits related to the amount and quality of data. Also, the price differential between streamers and OBN has decreased with the introduction of large, efficient multi-client OBN acquisition.
With full azimuth acquisition, structures and attribute variations can be properly understood, and complex geology can be illuminated in all directions. Considerable higher fold significantly improves the signal-to-noise ratio, as well as the ability to distinguish between structure and attribute anomalies.
Other advantages include ultra-long offsets, measurement of the full wavefield, and measurement of both P- and S-waves for better characterization of fluids in a reservoir.
OBN is also useful for 4D seismic (monitoring) – ocean bottom acquisition is not affected by tankers or platforms in the same manner as with streamer vessels. Ocean bottom acquisition can also be repositioned more easily than streamer or record continuously for days to months.
In summary, the Utsira OBN survey has resulted in several benefits for explorers and operators.
- The very long offsets have given better FWI velocity models, yielding more correct depth conversions.
- The high fold and full azimuths have given better illumination of the targets.
- The stable low frequencies, high signal-to-noise ratios, and multiple-free data have given better amplitude preservation and inversion results.
- The survey has already resulted in defining several new drillable leads.